Optical waveguide coupler circuit device

ABSTRACT

The present invention relates to an optical waveguide coupler device. As shown in FIG.  1,  the optical waveguide coupler device  8  comprises two optical waveguide cores  2, 3  formed on the surface of a substrate  1.  The optical waveguide cores  2, 3  are covered by lower and upper cladding layers  6, 7  both formed on the substrate  1;  and the two optical waveguide cores  2, 3  are brought close to and in parallel with each other at two sites to form two directional couplers  4, 5  there. The cross-section and refractive index of each of the optical waveguide cores  2, 3  and the parameters of other elements of the circuit are optimized such that an optical signal is reliably routed by the circuit without undergoing a wavelength shift even when the circuit is exposed to the changes of ambient temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymer-based optical waveguidecircuit stable to the changes of ambient temperature which is profitablyused as a wavelength division multiplexer or demultiplexer in, forexample, fiber optic communication.

2. Description of the Related Art

In a dense wavelength division multiplexing (DWDM) system applied tooptical carriers belonging to S, C and L bands, the wavelengthindependent waveguide coupler has been used that is capable of routingoptical carriers comprising 80-100 nm bands around 1550 nm into twochannels at a specified split ratio, being unaffected with theirwavelengths. The coupler that routes optical carriers at a split ratioof 1:1 is called a 3 dB coupler and has been used in multiple fiberoptic communication systems (K. Jinguji et al., “Mach-Zehnderinterferometer type optical waveguide coupler with wavelength-flattenedcoupling ratio,” Electron. Lett., 1990, Vol. 26, No. 17, pp. 1326-1327).

Such a wavelength independent waveguide coupler includes a Mach-Zehnderinterferometer type optical circuit 17 as shown in FIG. 8. The circuitcomprises two waveguide cores 13, 14 formed on the surface of asubstrate 20 prepared from a quartz or silicon wafer.

The waveguide cores 13, 14 are covered with lower and upper claddinglayers 18, 19 both of which are formed on the substrate 20. When thecore and cladding layers are mainly composed of silicon dioxide (SiO₂),the resulting optical circuit is called a quartz waveguide. When theyare composed of a polymer, the resulting optical circuit is called apolymeric waveguide.

The Mach-Zehnder interferometer type optical circuit 17 comprises twodirectional couplers 15, 16 which are obtained by bringing the twowaveguide cores 13, 14 close to and in parallel with each other. TheMach-Zehnder interferometer type optical circuit 17 receives an opticalsignal having a band width of 80-100 nm around 1550 nm either from aterminal 13 a or 14 a connected respectively to the waveguide core 13 or14 and splits the signal at a split ratio of 1:1 to deliver two outputseach having an intensity half that of the transmitted signal fromterminals 13 b, 14 b, or the other terminals of the waveguide cores 13,14.

An application of such a Mach-Zehnder interferometer type opticalcircuit includes an interleaver as shown in FIG. 2. This interleaverwith two polymeric waveguides 9, 10 comprises circuits 11, 12: each ofthe circuits is equivalent to the Mach-Zehnder interferometer typeoptical circuit 17 shown in FIG. 8, and the two circuits are arranged toeach other in a point symmetrical manner. If it is required to separatewavelengths having a bandwidth of 100 nm, it is only necessary for theinterleaver shown in FIG. 2 to be configured such that a proper opticalpath difference is inserted between the two waveguides 9, 10. Thissimplifies the designing of the interleaver.

For example, according to the interleaver shown in FIG. 2, it ispossible by designing the waveguides 9, 10 so as to produce a properoptical path difference, to receive optical signals λ1, . . . , λn eachhaving a band width of 80-100 nm around 1550 nm from a terminal 9 a of awaveguide 9, separate those signals according to their wavelengths,route them alternately into two channels, and deliver them as separateoutputs from the other terminals 9 b, 10 b of the waveguides 9, 10.

However, because the interleaver shown in FIG. 2 includes the waveguidesmade from a polymer, its optical characteristics are apt to vary in thepresence of changes of the ambient temperature, and thus the temperaturerange under which it can normally operate is rather limited. Thetemperature coefficient of the refractive index of a polymer materialused for the construction of the waveguides is ten or more times as highas that of quartz. Therefore, if the ambient temperature changes, thepolymeric waveguides 9, 10 and cladding layers covering those waveguideswill undergo a great change in their refractive indices; the parametersof the optical circuit including those waveguides and cladding layerswill also shift from the designed ranges, and the performance of theoptical circuit will depart from the designed level. Degradedperformance of an optical circuit with polymeric waveguides as a resultof the alteration of ambient temperature is mainly ascribed to thefollowing two reasons. The first reason is: a phase difference which isproduced as a result of properly chosen path length difference betweenthe two waveguides 9, 10 is modified in the presence of a change ofambient temperature, which in turn brings about a change in the centralwavelength of the affected optical carrier. The second reason is: anoptical carrier having passed the Mach-Zehnder interferometer typeoptical circuits 17 exposed to a change of ambient temperature ismodified such that its wavelength shifts to a shorter or longer one. Forexample, the refractive index of a polymeric waveguide has a temperaturecoefficient of about−(1.1-1.8)×10⁻⁴/K, and if such a polymeric waveguideis exposed to a temperature change of 40° C., a carrier having passedthrough the waveguide will undergo a shift of about 6.5 nm in itswavelength. Assume that an interleaver with polymeric Mach-Zehnderinterferometer type optical circuits separates carriers at 0.8 nmintervals and is exposed to a temperature change of 40° C., and thus acarrier having passed the Mach-Zehnder interferometer type opticalcircuits suffers a shift of about 6.5 nm in its wavelength. Then, thecarrier will be routed to a channel by eight channels shifted from theone to which it should be routed. According to an experiment, if such aninterleaver as above is exposed to a temperature change of 40° C., acarrier having passed through the Mach-Zehnder interferometer typeoptical circuits 11, 12 is modified so much that its output changes by±2% or more at the ends of its band width, and that it is impossible tomaintain the crosstalk between adjacent carriers at −30 dB or lower.

The first problem will be solved by adjusting the physical parameters ofwaveguides 9, 10 such that they satisfy the following equation for awavelength having a given bandwidth:

∂β₁₀ /∂T=∂β ₉ /∂T(L ₉ /L ₁₀)  (1)

where β₉ and β₁₀ represent the transmission constants of waveguides 9and 10 for a mode of optical carriers, L₉ and L₁₀ the lengths of lightpath of the waveguides 9, 10 enclosed by the Mach-Zehnder interferometertype optical circuits 11, 12, and T temperature.

For solving the second problem, it is necessary to redesign the overallstructure of the interleaver because its Mach-Zehnder interferometertype optical circuits 11, 12 are too complex in their structure. Similarproblems to the above are also observed in certain types of quartzoptical waveguide circuits. The refractive index of a quartz materialhas a positive temperature coefficient whose absolute value is smallerthan that of a corresponding polymeric material. Therefore, a knownmethod for preparing an optical waveguide circuit from quartz consistsof covering a quartz waveguide core with a polymeric coat whoserefractive index has a negative temperature coefficient sufficientlylarge to cancel the positivity of the temperature coefficient of thequartz waveguide core. However, generally a polymeric material has arefractive index whose temperature coefficient has too large a negativevalue to cancel the positivity of the temperature coefficient of aquartz material. Naturally, this method can not be applied for thepolymeric optical waveguide circuit here concerned.

A known method for compensating for the thermal characteristics of apolymeric waveguide core is to employ a substrate made from a polymerhaving a high thermal expansion. To put it more specifically, thismethod consists of employing a polymeric substrate which has a thermalexpansion sufficiently high to cancel the negative temperaturecoefficient of the refractive index of a polymeric waveguide core.However, a substrate made from quartz or silicon generally has a lowthermal expansion, and thus as far as based on this method, it will notbe possible to integrate optical waveguide circuits on a siliconsubstrate as in the conventional electronic technology wheresemiconductor devices are integrated on a silicon substrate.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide aMach-Zehnder interferometer-based polymeric waveguide circuit unaffectedwith the change of ambient temperature which is obtainable by arrangingwaveguides on a silicon or quartz substrate using conventional ICtechnology, while maintaining the advantage of low production cost whichis the most important impetus for the introduction of polymeric opticalwaveguide circuits.

An object of the present invention is to provide an optical waveguidecoupler circuit device comprising a substrate; a polymeric lowercladding layer formed on the substrate; at least two polymeric opticalwaveguides formed on the polymeric lower cladding layer; a polymericupper cladding layer covering the optical waveguides; and pluraldirectional couplers which are obtained by choosing any pair from the atleast two optical waveguides, and bringing them close to each other atplural sites, wherein the two paired optical waveguides are configuredsuch that the difference between their effective optical paths spanningbetween arbitrarily chosen adjacent directional couplers is defined asΔL and ΔL=0.6 to 0.8 μm and each of the plural directional couplerscomprises a parallel section at which the two optical waveguides aredisposed in parallel with each other.

Another object of this invention is to provide the optical waveguidecoupler circuit device wherein the polymeric optical waveguide is madefrom a polymer having a refractive index of 1.5182 to 1.5667.

Another object of this invention is to provide the optical waveguidecoupler circuit device wherein the polymeric lower cladding layer ismade from a polymer having a refractive index of 1.5136 to 1.5620.

A further object of this invention is to provide the optical waveguidecoupler circuit device wherein the polymeric upper cladding layer ismade from a polymer having a refractive index of 1.5136 to 1.5620.

A further object of this invention is to provide the optical waveguidecoupler circuit device wherein the length of the two optical waveguidesof one directional coupler is chosen to be 0.031 to 0.072 mm while thelength of the two optical waveguides of the other directional coupler ischosen to be 0.982 to 1.741 mm.

A further object of this invention is to provide the optical waveguidecoupler circuit device wherein the gap between two parallel runningwaveguides is chosen to be 4.1 to 6.4 μm for both directional couplers.

A further object of this invention is to provide the optical waveguidecoupler circuit device wherein each of the optical waveguides isconfigured to have an oblong cross-section having a width w and athickness t.

A further object of this invention is to provide the optical waveguidecoupler circuit device wherein each of the optical waveguides isconfigured to have a square cross-section.

A further object hereof is to provide the optical waveguide couplercircuit device wherein each of the optical waveguides is configured tohave a square cross-section with a side of 6 to 8 μm.

A further object hereof is to provide the optical waveguide couplercircuit device wherein the substrate is made of a quartz plate.

A further object hereof is to provide the optical waveguide couplercircuit device wherein the substrate is made of a silicon plate.

A further object hereof is to provide the optical waveguide couplercircuit device wherein the substrate is made of a polyimide resin plate.Other objects and advantages will become apparent as this disclosureproceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flat and lateral view of a polymeric optical waveguidecoupler device representing an embodiment of this invention.

FIG. 2 shows a flat view of an interleaver as an application of thepolymeric optical waveguide circuit.

FIG. 3 shows a graph representing the split ratio of the embodimentshown in FIG. 1 to an optical signal when the wavelength of the opticalsignal is varied and the ambient temperature is kept at 20° C.

FIG. 4 shows a graph representing the split ratio of the embodimentshown in FIG. 1 to an optical signal when the wavelength of the opticalsignal is varied and the ambient temperature is kept at 0° C.

FIG. 5 shows a graph representing the split ratio of the embodimentshown in FIG. 1 to an optical signal when the wavelength of the opticalsignal is varied and the ambient temperature is kept at 10° C.

FIG. 6 shows a graph representing the split ratio of the embodimentshown in FIG. 1 to an optical signal when the wavelength of the opticalsignal is varied and the ambient temperature is kept at 30° C.

FIG. 7 shows a graph representing the split ratio of the embodimentshown in FIG. 1 to an optical signal when the wavelength of the opticalsignal is varied and the ambient temperature is kept at 40° C.

FIG. 8 shows a top and lateral view of a Mach-Zehnder interferometertype optical waveguide circuit 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow with reference to attached figures.

A polymeric optical waveguide coupler circuit device 8 stable to thechanges of ambient temperature representing an embodiment of thisinvention and shown in FIG. 1 comprises two optical waveguide cores 2, 3formed on the surface of a substrate 1 prepared from a quartz plate, asilicon wafer or a polyimide resin plate and the like. The opticalwaveguide cores 2, 3 are covered by a lower cladding layer 6 and anupper cladding layer 7 both being formed on the substrate 1. The device8 further comprises two directional couplers 4, 5 each of which isobtained by bringing the two optical waveguides 2, 3 close to and inparallel with each other. With the optical circuit device 8, thecross-sections and refractive indices of the optical waveguide cores 2,3, and other circuit parameters are optimized such that the performanceof the device is maintained even when light passing through the deviceundergoes a shift in its wavelength, or the ambient temperature ischanged.

How this is achieved will be described below.

Referring to FIG. 1, assume that light enters from a terminal 2 a of theoptical waveguide core 2 into the optical circuit 8 of FIG. 1; and thelight is split into two components with a split ratio of η which arethen delivered as outputs from the other terminals 2 b, 3 b of theoptical waveguides 2, 3. Then, the split ratio η expressed in power canbe expressed by the following equation: $\begin{matrix}{{\eta = {\frac{\left| B_{out} \right|^{2}}{\left| A_{out} \middle| {}_{2}{+ \left| B_{out} \right|^{2}} \right.} = {a^{2} + b^{2} + {2\quad {ab}\quad {\cos \left( {{\beta\Delta}\quad L} \right)}}}}}{where}} & (2) \\{a = {{\cos \left\lbrack {\frac{\pi}{2L_{c1}}\left( {L_{1} + L_{e1}} \right)} \right\rbrack}{\sin \left\lbrack {\frac{\pi}{2L_{c2}}\left( {L_{2} + L_{e2}} \right)} \right\rbrack}}} & (3) \\{b = {{\sin \left\lbrack {\frac{\pi}{2L_{c1}}\left( {L_{1} + L_{e1}} \right)} \right\rbrack}{{\cos \left\lbrack {\frac{\pi}{2L_{c2}}\left( {L_{2} + L_{e2}} \right)} \right\rbrack}.}}} & (4)\end{matrix}$

In the above equations, A_(out) and B_(out) represent the amplitudes ofthe light waves delivered as outputs from the terminals 2 b, 3 b of theoptical waveguide cores 2, 3; L_(c1) and L_(c2) the total couplinglengths of the directional couplers 4, 5; L₁ and L₂ the lengths of theparallel sections of the directional couplers 4,5; L_(e1), and L_(e2)the equivalent incremental lengths of the parallel sections of thedirectional couplers 4, 5; β the transmission constant of waveguides 9and 10 for a mode of light waves; and ΔL the optical path differencebetween the two optical waveguide cores 2, 3.

L_(c1) and L_(c2), and L_(e1) and L_(e1) can be expressed as a functionof the following parameters: λ, the wavelength of a light wave; w and tthe width and thickness of the optical waveguide cores 2, 3; n_(g) therefractive index of the optical waveguide cores 2, 3; n_(c) therefractive index of the cladding layers 6, 7; and s₁ and s₂ gaps of theparallel sections of the directional couplers 4, 5. Therefore,

L _(ci) =L _(ci)(λ, w, t, n _(g) , n _(c) , s _(i)), i=1, 2  (5)

L _(ei) =L _(ei)(λ, w, t, n _(g) , n _(c) , s _(i)), i=1, 2  (6)

The transmission constant β can be expressed as a function of thefollowing parameters: λ, the wavelength of a light wave; w and t thewidth and thickness of the optical waveguide cores 2, 3; n_(g) therefractive index of the optical waveguide cores 2, 3; and n_(c) therefractive index of the cladding layers 6, 7, as follows.

β=β(λ, w, t, n _(g) , n _(c))  (7)

The refractive index n_(g) of the optical waveguide cores 2, 3, and therefractive index n_(c) of the cladding layers 6, 7 can be expressed as afunction of ambient temperature T and the wavelength λ of a light wave,as follows.

n _(g) =n _(g)(λ, T)  (8)

n _(c) =n _(c)(λ, T)  (9)

For a given light wave having a certain band width operating under agiven temperature range, the width w and thickness t of the opticalwaveguide cores 2, 3; gaps s₁ and s₂ of the parallel sections of thedirectional couplers 4, 5; the refractive index n_(g) of the opticalwaveguide cores 2, 3; the refractive index n_(c) of the cladding layers6, 7; the lengths L₁, L₂ of the parallel sections of the directionalcouplers 4,5; and the optical path difference ΔL between the two opticalwaveguide cores 2, 3 are properly chosen in order to satisfy thefollowing simultaneous equations:

η(λ, T)=50%±δη  (10)

$\begin{matrix}{\left| {{\delta\eta}\left( {\lambda,\quad T} \right)} \right|_{\lambda} = \left| {\frac{\partial\eta}{\partial\lambda} \times {\delta\lambda}} \middle| {< \sigma_{\lambda}} \right.} & (11) \\{\left| {{\delta\eta}\left( {\lambda,\quad T} \right)} \right|_{T} = \left| {\frac{\partial\eta}{\partial T} \times \delta \quad T} \middle| {< \sigma_{T}} \right.} & (12)\end{matrix}$

 δη=|δη(λ, T)|_(λ)+|δη(λ, T)|_(T)  (13)

The resulting optical waveguide circuit will split a light wave at aspecified split ratio independent of its wavelength, and beingunaffected by the ambient temperature.

In the above calculation, σλ and σT are chosen so as to make δη≦1%.

The polymeric optical waveguide coupler circuit 8 stable to ambienttemperature changes produced by the above method according to thisinvention has its parameters optimized as described below.

With regard to the optical circuit device as shown in FIG. 1, itssubstrate 1 is made from quartz, silicon or a polyimide resin and thelike; the optical waveguides 2, 3 from a polymer having a refractiveindex n_(g) of 1.5182 to 1.5667; and the lower and upper cladding layers6, 7 for covering the optical waveguides 2, 3 from another polymerhaving a refractive index n_(c) of 1.5136 to 1.5620. Each of the opticalwaveguides 2, 3 has a square cross-section with a side of 6 to 8 μm; theparallel section of the directional coupler 4 has a length L₁ of 0.031to 0.072 or 0.982 to 1.741 mm; the parallel section of the directionalcoupler 5 has a length L₂ of 0.982 to 1.741 or 0.031 to 0.072 mm; thegap of the parallel waveguides in the directional coupler 4 has a sizeS₁ of 4.1 to 6.4 μm; the gap of the parallel waveguides in thedirectional coupler 5 has a size S₂ of 4.1 to 6.4 μm; and the opticalpath difference ΔL between the two optical waveguides 2, 3 is 0.6 to 0.8μm.

Examples of the present invention will be described below with referenceto attached figures.

EXAMPLE

FIG. 1 shows an optical waveguide coupler circuit device of thisinvention.

The polymeric optical waveguide coupler circuit device 8 stable to thechanges of ambient temperature comprises two optical waveguide cores 2,3 formed on the surface of a substrate 1 made of a quartz plate as shownin FIG. 1. The optical waveguide cores 2, 3 are made from a polymermaterial having a refractive index n_(g) of 1.5182 to 1.5667 as shown inthe figure, and has a square cross-section with a side of 6 to 8 μm. Thematerial constituting the lower and upper cladding layers 6, 7 coveringthe optical waveguide cores 2, 3 is a polymer having a refractive indexn_(c) of 1.5136 to 1.5620.

The optical waveguide cores 2, 3 form two directional couplers 4, 5 byapproaching to and running in parallel with each other at two sites. Forthe directional coupler 4, the length L₁ of the parallel section is0.031 to 0.072 mm, and the gap S₁ between the parallel runningwaveguides is 4.1 to 6.4 μm. For the directional coupler 5, thecorresponding length L and gap S₂ are 0.982 to 1.741 mm and 4.1 to 6.4μm, respectively.

Needless to say, the values of L₁ and L₂ may be exchanged for with eachother, that is, L₁ may take 0.982 to 1.741 mm and L₁ may take 0.031 to0.072 mm.

The optical waveguide cores 2, 3 are configured such that the differencebetween their optical path lengths falls within 0.6 to 0.8 μm.

An exemplary method for fabricating the polymeric optical waveguidecoupler circuit device 8 stable to the change of ambient temperature ofthis invention will be described below.

A quartz plate to serve as a substrate 1 is prepared. A solution of apolymer which will form a lower cladding layer is prepared by dissolvingthe same molecular amounts of 4,4′-(hexafluoroisopropylidene)diphthalicanhydride and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl inN,N-dimethylacetamide, and the solution is stirred at 25° C. for 24hours in the presence of nitrogen. The resulting solution is applied byspin coating onto the substrate 1 to form a coat thereupon. The assemblyis removed of the solvent, and subjected to a heating treatment, to forma polymeric lower cladding layer 6 having a thickness of about 20 mm onthe substrate 1.

A solution of another polymer is prepared as follows: part of diaminethat has been used for producing the polymer responsible for theformation of the lower cladding layer, that is, part of2,2′-bis(trifluoromethyl)-4,4′-diamonobiphenyl is substituted foranother diamine, that is, 4,4′-diaminodiphenyl ether, and the twodiamines are added to 4,4′-(hexafluoroisopropylidene)diphthalicanhydride such that the summed molecular amounts of the two diamines areequal to the molecular amount of the latter. The resulting solution istreated as above, and is applied by spin coating onto the above assemblyto form a coat on the coat to serve as the lower cladding layer. Theassembly is removed of the solvent, and subjected to a heatingtreatment, to form a polymeric core layer with a thickness of about 8 μmwhich has a refractive index higher by about 0.25 to 0.45% than that ofthe lower cladding layer 6.

A specified optical waveguide pattern is formed via a photo-resist onthe surface of the core layer. The core layer is pattern-etched viareactive ion etching in the presence of oxygen gas, to form opticalwaveguide cores 2, 3 having designed configurations. Then, the samepolymer solution as is used for the formation of the lower claddinglayer is applied by spin coating to the assembly. The resulting assemblyis removed of the solvent and subjected to heating treatment. Thus, anupper cladding layer 7 is formed over the optical waveguide cores 2, 3to embed the latter. The upper cladding layer 7 must have a refractiveindex higher than that of the waveguide cores, but needs not to have thesame refractive index with that of the lower cladding layer 6.

The above example is related with a substrate 1 made of a quartz plate.The substrate 1 may be made of a silicon plate or a polyimide plate. Theoptical waveguide core may have an oblong cross-section with a width wand thickness t.

A performance test was conducted on the above exemplary polymericoptical waveguide coupler circuit device 8 stable to the changes ofambient temperature. The test results are as follows.

In an environment where the temperature was kept at 20° C., thepolymeric optical waveguide coupler circuit 8 stable to the changes ofambient temperature as shown in FIG. 1 received seven light waves, thatis, light waves with the wavelengths of 1490, 1510, 1530, 1550, 1570,1590 and 1610 nm from a terminal 2 a of the optical waveguide core 2.The light waves were split during their passage through the circuit anddelivered as outputs from the opposite terminals 2 b, 3 b of the opticalwaveguide cores 2, 3. For each light wave, the ratio of its poweroutputs from the two terminals (power split ratio) was determined, andthe power split ratio was plotted as a function of wavelength as seen inFIG. 3. The power split ratio for any wavelengths enclosed in the bandwidth of 120 nm is in the range of 50±0.69%.

Then, the ambient temperature was changed to 0, 10, 30, and 40° C., andthe measurement as had been performed on the device kept at 20° C. wasrepeated. For each temperature, the power split ratio was plotted as afunction of wavelength. The results are shown in FIGS. 4, 5, 6 and 7. Asseen from the figures, the power split ratios are in the range of50±0.67, 50±0.70, 50±0.69, and 50±0.68% respectively. They were hardlyaffected by the changes of ambient temperature.

As seen from FIGS. 3 and 7, the two wavelengths at the ends of thebandwidth, that is, 1490 and 1610 nm showed scarcely any notablevariations in their power split ratios even when the ambient temperaturewas changed from 0° C. to 40° C., that is, the maximum variationsactually observed were only 0.06 and 0.002% respectively. There wasscarcely any band shift in the presence of a notable change of ambienttemperature.

As seen from above, with the polymeric optical waveguide coupler circuit8 of this invention stable to the changes of ambient temperature,optical signals falling within the bandwidth of 120 nm with 1550 nm attheir central wavelength which are used in fiber optic communicationbased on dense wavelength division multiplexing are reliably routedbecause their wavelength being hardly modified during their passagethrough the circuit, even when the ambient temperature is changed from0° C. to 40° C. Accordingly, the polymeric optical waveguide couplercircuit 8 of this invention stable to the changes of ambient temperaturemay serve, as one of its applications, as thermally stable couplers 11,12 constituting a polymeric optical waveguide interleaver as shown inFIG. 2.

Needless to say, the use of the circuit of this invention is not limitedto a polymeric optical waveguide interleaver. For example, the circuitsof this invention may be connected to two 1×N channel optical waveguidesplitters to produce a 2×2N channel optical waveguide splitter stable tothe changes of ambient temperature.

According to this invention, the cross-sections and refractive indicesof the waveguides and the parameters of other circuit elementsconstituting the polymeric optical waveguide coupler circuit device areoptimized such that light passing through the device can reliably keepits wavelength unaffected by the changes of ambient temperature. Theeffect of the change of ambient temperature on the power split ratio ofthe device which would be otherwise manifest is obviously minimized.

As seen from above, with the polymeric optical waveguide coupler circuitdevice of this invention, optical signals falling within a bandwidth of120 nm with 1550 nm at its central wavelength which are used in fiberoptic communication based on dense wavelength division multiplexing arereliably routed even when the ambient temperature is changed from 0° C.to 40° C., without requiring a certain heat-insulating means.

Because the optical waveguide circuit device of this invention isoptimized as described above, even wavelengths falling at the ends ofthe bandwidth are hardly affected by the change of ambient temperature,and there is no band shift either. Thus, it may be used, as one of itsapplications, as a thermally stable polymeric optical waveguideinterleaver.

Because the method provided by the present invention optimizes theoptical waveguide coupling circuit device using the same materials asused in the conventional optical waveguide device, it is not necessaryto alter the conventional fabrication processes themselves. Thus, themethod of this invention, maintaining the advantage of the conventionalmethod of producing a polymeric optical waveguide device, that is, thelow production cost, ensures the production of an optical waveguidecircuit device stable to the changes of ambient temperature.

What is claimed is:
 1. An optical waveguide coupler circuit devicecomprising: a substrate; a polymeric lower cladding layer formed on thesubstrate; at least two polymeric optical waveguides formed on thepolymeric lower cladding layer; a polymeric upper cladding layercovering the optical waveguides; and plural directional couplers whichare obtained by choosing any pair from the at least two opticalwaveguides, and bringing them close to each other at plural sites,wherein: each of the optical waveguides has two ends, one end serving asan input terminal and the other as an output terminal; the two pairedoptical waveguides are configured such that the difference between theireffective optical paths spanning between arbitrarily chosen adjacentdirectional couplers is defined as ΔL and ΔL=0.6 to 0.8 μm, each of theplural directional couplers comprises a parallel section at which thetwo optical waveguides are disposed in parallel with each other, thepolymeric optical waveguide is made from a polymer having a refractiveindex of 1.5182 to 1.5667, the polymeric lower cladding layer is madefrom a polymer having a refractive index of 1.5136 to 1.5620, thepolymeric upper cladding layer is made from a polymer having arefractive index of 1.5136 to 1.5620, the length of the two opticalwaveguides of one directional coupler is chosen to be 0.031 to 0.072 mmwhile the length of the two optical waveguides of the other directionalcoupler is chosen to be 0.982 to 1.741 mm, and the gap between twoparallel running waveguides is chosen to be 4.1 to 6.4 μm for bothdirectional couplers.
 2. An optical waveguide coupler circuit device asdescribed in claim 1 wherein each of the optical waveguides isconfigured to have an oblong cross-section having a width w and athickness t.
 3. An optical waveguide coupler circuit device as describedin claim 1 wherein each of the optical waveguides is configured to havea square cross-section.
 4. An optical waveguide coupler circuit deviceas described in claim 1 wherein each of the optical waveguides isconfigured to have a square cross-section with a side of 6 to 8 μm. 5.An optical waveguide coupler circuit device as described in claim 1wherein the substrate is made of a quartz plate.
 6. An optical waveguidecoupler circuit device as described in claim 1 wherein the substrate ismade of a silicon plate.
 7. An optical waveguide coupler circuit deviceas described in claim 1 wherein the substrate is made of a polyimideresin plate.